In a telephone system, there can be many circumstances where it would be desirable to be able to detect when an extension phone is off-hook. For example, during a data transmission to a PC, it may be desirable for the PC to be able to detect if an extension goes off-hook so as to stop transmission or take other measures to preserve data integrity. Also, in the Analog Display Services Interface (ADSI) protocol, a requirement of the protocol is that an ADSI phone instrument must be able to detect at all times the use of other extension phones when the ADSI phone is either on-hook or off-hook. When the ADSI phone detects that an extension is off-hook, the ADSI phone must not start an ADSI data transmission, but may finish receiving any ADSI transmission. This reduces the chances of inflicting loud and harsh signal noise from the handset upon the user of an extension phone, and also makes the phone line available for potential emergency calls. Also, it may be desirable to know if someone may be eavesdropping on an extension phone.
Each of the above examples, and in fact many more applications, is currently available as a computer integrated telephony (CIT) application wherein a general purpose computer, such as a PC, having a telephony interface interacts with the telephone network to provide the desired functionality.
Typically, detection of the hook state of an extension phone is accomplished by monitoring the voltage or current between the tip and ring of the phone line.
In the telephony interfaces of many CIT systems, the ability to monitor the voltage or current between the tip and ring is not provided. However, it is common for the telephony interface to have signal generating and detection capabilities. These telephony interfaces typically are able to generate and detect DTMF and FSK in-band signals. This signal generating and detection capability is usually manifested in a digital signal processor (DSP), which typically can be programmed to generate and detect a wide spectral range of signals.
Accordingly, it is an object of the present invention to provide a CIT system that has the ability to detect when an extension phone is off-hook by using the signal generating and detection capabilities found on a typical telephony interface card.